1. Field of the Invention
This invention relates to the field of signal correction in a communication system, and, more particularly, to precorrection of the phase of an information-carrying constant envelope signal without introducing amplitude modulation to the signal.
2. Description of the Prior Art
One difficulty in transmitting a constant envelope signal (e.g., frequency or phase modulated (FM/PM) signals) is that any linear distortion on the transmitted signal results in non-linear distortion at the receiver. For example, in an FM radio signal, significant audio distortion and degraded stereo separation can result from even fairly minute linear distortion. One source of linear distortion comes from linear filtering effects occurring downstream of a power amplifier. If the filtering effects pass a relatively wide band of frequencies, the filtering effects will add very little distortion to the signal and can be ignored for most applications. Unfortunately, not all sources of post-amplification linear distortion have a wide frequency passband.
For example, in an application where a constant envelope signal needs to be combined with other signals into a common antenna, some form of radio frequency (RF) combining system is generally utilized. The RF combining system, referred to as a filterplexer, can be modeled, in general, as a linear filter that produces in-band distortion and a band limiting of the output signal. This manifests itself as amplitude error and group-delay/phase error. For a phase modulated signal, the phase component of this in-band response will result in signal errors (e.g., audio distortion in FM broadcasting and symbol errors in a BPSK/QPSK system) at one or more associated receivers. This distortion is generally compensated for at an equalizer at the filtering element, but practical equalizers can only partially mitigate the distortion. For high power broadcast transmitters, even this partial mitigation requires physically large, expensive, high power equalizers. These equalizers require manual tuning to account for temperature variations and other sources of drift, representing an additional complication in the transmission process.